The present invention relates to a vehicle parameter monitoring system and particularly a tire pressure monitoring system using radio frequency signals to communicate the detected tire pressure from the tires to a message center of the vehicle.
Excessive tire wear resulting in premature tire failure frequently results from under inflation of tires. Recent advances in tire manufacturing technology have produced tires that may be used for several miles at extremely low pressures without notice by the vehicle operator. Further, when such tires are at low pressure, the tires are not to be used above specified speeds. Thus, because a vehicle operator may not realize that a tire is nearly flat, these advances have increased the need for a system capable of monitoring tire pressures and warning a vehicle operator when the tire pressure is low.
In order to sense the pressure inside a tire, tire pressure monitoring systems utilize a pressure sensor located within the tire. The difficulty in implementing such systems is determining how to communicate the output of the pressure sensor located within a rotating tire to a warning display typically located in a fixed position in the vehicle's interior.
U.S. Pat. No. 5,285,189 discloses a tire pressure monitoring system that provides one solution to the above problem by utilizing radio frequency (RF) signals transmitted from transmitters located within the tires to a dedicated external receiver. In such systems, the transmitters typically include microcontrollers coupled to the pressure sensors for generating and transmitting encoded radio signals at the same carrier frequency. The encoded radio signals include tire pressure information and transmitter device identification information, which is useful for identifying from which tire the tire pressure information is transmitted. The receiver of this system receives the encoded radio signal and decodes the transmitted information to provide the vehicle operator an alarm if low tire pressure is detected.
One problem with the above-described system is the excessive cost involved in implementing such a system in a vehicle. Further, by placing separate microcontrollers in each tire, not only are the energy demands for the tire-mounted sensor increased, but the potential for damage to the sensor is also increased.